1. Field of the Invention
This invention relates to an electrical switch for use in switching to a bypass circuit of a liquid circulation system, such as an hydraulic system or a lubrication system for circulating lubricating oil. More particularly, this invention relates to a pressure differential switch that self-actuates as a result of an excessive pressure differential in a liquid circulation system and has a magnetically actuated thermal lockout feature.
2. Description of the Prior Art
U.S. Pat. No. 4,130,745, to Hetzer, discloses a pressure differential detector switch utilizing a piston which is free to travel within a housing. One side of the piston is exposed to the high peessure side of a pressure differential application, and the other side is exposed to the low pressure side. A spring is provided in the housing to resiliently urge the piston toward the high pressure side, in cooperation with the pressure on the low pressure side. When the force on the piston resulting from pressure differential between the high pressure side and the low pressure side exceeds the force on the piston resulting from the spring, the pressure differential force will move the piston away from the high pressure side to compress the spring. The piston carries a permanent magnet, and a reed switch is positioned on the outside of the housing to be magnetically actuated by the change in position of the piston resulting from an excessive pressure differential condition. A reed switch of the type utilized in the device disclosed in this patent has lmited current carrying capacity and a high resonant frequency due to its low moving mass and high spring constant.
U.S. Pat. No. 4,645,887, to Whiting, discloses a pressure differential bypass sensor switch. A housing is provided having two perpendicularly oriented legs, each leg having an axial passage. A reciprocable piston is mounted in one of the legs and an axial travel switch in the other. One end of the piston is exposed to the high pressure side of the fluid system and the other end of the piston is exposed to the low pressure side of the fluid system. A spring is used to bias the piston against the force caused on the piston by the differential in fluid pressure. A permanent magnet is located at the high pressure end of the piston. When the piston moves due to the force of the pressure differential exceeding the biasing force of the spring, one of the poles of the magnet moves to adjacency with the axial travel switch, causing it to actuate. Although the axial travel switch is an improvement over the reed switch, there is no provision in this invention for locking out false indications due to the higher viscosity of the system fluids when operating at low temperature.
U.S. Pat. 4,266,517, to Sakakibara et al, discloses a differential pressure switch for controlling internal combustion engines. A housing is provided which supports, by a diaphragm, a movable member. The movable member is exposed to pressure differentials existing in the fluid system on either side of the diaphragm. A spring biases the movable member against the force caused by the differential fluid pressure acting on the movable member. A permanent magnet covered with a thermal ferrite is provided on the movable member. When the movable member moves due to the force of the pressure differential exceeding the biasing force of the spring, the magnet aligns with a reed type electrical switch. Through induced magnetization in the thin blades of the reed switch, the blades are caused to contact, closing the switch. The thermal ferrite is selected to be very magnetic when below its Curie temperature, a temperature that corresponds to a high viscosity in the system fluid. Above the Curie temperature, the thermal ferrite is substantially non-magnetic, corresponding to temperatures with low viscosity in the system fluid. When the system fluid is below the Curie temperature, the thermal ferrite confines the magnetic field, preventing the magnet from actuating the reed switch blades. This invention suffers from the same problems experienced by Hetzer because a reed switch is used. Further, the use of a diaphragm mounted piston structure and the necessity that the thermal ferrite be moved with the magnet, could cause the system to suffer from insensitivity and hysteresis in response to changes in fluid pressure differential.